The present invention relates to a magnetic recording medium and, in particular, a magnetic recording medium for use in a hard disk apparatus and a method for manufacturing the same.
The magnetic disk apparatus, in particular, a hard disk apparatus using a magnetic recording medium with a magnetic film formed on a rigid disk-like substrate, has often been used as an external memory apparatus for a personal computer, large-sized computer and computer apparatus for word processors, etc., in view of the high recording density, high-speed writable feature and low bit cost, etc., and is highly demanded to further improve the magnetic density for obtaining large recording capacity. In order to enhance the magnetic density, there is a demand for the improvement of a line recording resolution by a magnetic film's high coercive force in the magnetic recording medium. In order to prevent the occurrence of any playback error, there is also a demand for reducing noise in a playback output and improving the quality of the played-back signal.
It has been known that the noise in the playback output from the magnetic recording medium has a relation to the structure of the magnetic recording medium's magnetic film and is increased in the case where a strong magnetic interaction acts between magnetic crystal grains in a ferromagnetic substance constituting the magnetic film. In order to reduce the noise and enhance the quality of the played-back signal, various designs have been attempted to reduce the magnetic interaction between the magnetic crystal grains.
The first method is by enhancing the concentration of Cr added to the magnetic film and making the boundary non-magnetized. The second method is by segregating oxygen at a boundary under the film formation conditions of the magnetic film and the third method is by distributing magnetic crystal grains in a non-magnetic matrix and decreasing the magnetic interaction. Further, it has been considered that a static magnetic interaction is also decreased between the magnetic crystal grains due to the addition of Cr reducing a saturated amount of magnetization of the magnetic crystal grains.
The second and third methods can achieve a higher coercive force without lowering the magnetic anisotropy in the crystal and can realize a high recording density.
Further it has also been known that an activated magnetic moment v.multidot.Isb corresponding to the amount of magnetization of magnetically separated magnetic crystal grains has a correlation to the noise amount. As the method for controlling the v.multidot.Isb and lowering the noise amount, attempts have been made to decrease the film thickness of the magnetic film. As a result of studies made by the inventors on the case where an error rate is measured on a medium with a magnetic film of reduced thickness, that is, a medium combined with an associated head, it has been found impossible to obtain an improved error rate. It has been considered that the reduced thickness of the magnetic film leads to a fall in the intensity of a played-back signal and never leads to an improved quality of the played-back signal.
Further, the increase of Cr added to the magnetic film also results in a fall in the activated magnetic moment v.multidot.Isb. In that case, there also occur a fall in the intensity of a played-back signal and in the quality of the error rate.
As set out above, even if it is possible for the conventional technique to lower the noise in the playback output, there occur a fall in the noise and a fall in the intensity of the played-back signal. It is, therefore, not possible to improve the quality of the played-back signal and effectively to improve the error rate.
Further, even in the above-mentioned first and second methods, if the size of the magnetic crystal grain is reduced, the recorded magnetized state changes with time due to the effect of a thermal fluctuation involved and the noise level is increased, thus leading to an increase in the error rate upon playback. And with the non-magnetization of the boundary the magnetized amount of the magnetic film is decreased, so that the playback output is lowered.
In this way, in the conventional method for non-magnetizing the boundary so as to lower the noise in the output played-back by the head from the magnetic recording medium, the size of the magnetic crystal grains is decreased when the S/N ratio is improved as desired, so that a playback error is increased with the lapse of time due to the effect of thermal fluctuation. Further, the playback output level is also lowered with a volume increase in the non-magnetic boundary.
In the magnetic recording medium, it becomes important that, in order to enhance the line recording density, the width of the magnetization inversion be decreased and the noise of the medium be lowered.
In order to decrease the width of the magnetization inversion it is considered effective to increase the coercive force (Hc) of the medium. With the present hard disk apparatus, therefore, use is made of a magnetic thin-film medium whose Hc is of the order 2,000e. Although the Hc of the medium is currently restricted by the recording capability of the magnetic head, it is essential that, with an enhancing recording capability of the magnetic head, there be a growing demand for a high coercive force (Hc) of the medium.
Some prior arts are known which aim at a high Hc and low noise in the magnetic recording medium.
The following are the known techniques regarding the achievement of a higher coercive force Hc, a lower noise level and a higher playback output:
(1) Under the title "Sputtered Multilayer-Films For Digital Magnetic Recording" of IEEE Transactions on Magnetics: Vol. 15. No. 3. P. 1135, issued July 1979, there appears a description to the effect that a cobalt single layer involves a decrease in a coercive force with an increasing film thickness, but that the cobalt/chromium stacked layer involves a substantially constant coercive force with an increase in a total film thickness.
(2) JPN PAT APPLN KOKAI PUBLICATION No. 63-146219 proposes forming a magnetic recording layer of a thin-film medium by a plurality of magnetic layers and inserting a non-magnetic intermediate layer between the adjacent magnetic layers whereby a multilevel magnetic layer type magnetic recording medium is provided with the magnetic coupling between the magnetic layers being lowered and the medium-causing noise being lowered.
(3) JPN PAT APPLN KOKAI PUBLICATION No. 61-34721 proposes a magnetic recording medium with a CoCr layer stacked on a CoPt layer. In the proposed medium, the overlying CoCr layer is formed on the underlying CoPt layer in a better orientation state and, at the same time, an adequate magnetic characteristic is exhibited with the CoPt layer as an intra-plane recording layer, so that, as a whole, a playback output is improved in a long wavelength range while exhibiting an advantage of a vertical recording system in a short wavelength range.
(4) JPN PAT APPLN KOKAI PUBLICATION No. 61-194635 proposes forming an alternate layer structure of a Co thin film and Pt thin film, that is, a layer structure of two or more thin films, and subjecting the layer structure to heat treatment to provide a thin film permanent magnet.
(5) JPN PAT APPLN KOKAI PUBLICATION No. 62-257616 proposes a vertical magnetic recording medium in which a multi-level magnetic thin film structure for vertical magnetic recording includes a non-magnetic nucleus-forming thin film having a substantially hexagonal packed crystal structure and a magnetic thin film formed of an alloy of cobalt and an element selected from the group consisting of platinum, nickel, rhenium and palladium. It was reported there that, with the use of such a structure, it was possible to enhance a coercive force which would otherwise be inadequate for a single thin film and to obtain a practically applicable vertical magnetic recording medium.
(6) JPN PAT APPLN KOKAI PUBLICATION No. 02-210614 proposes a magnetic recording medium provided by forming a mutilevel structure with two or more CoCrPt magnetic thin films and inserting a non-magnetic thin film between the respective magnetic thin films and, by doing so, making a playback output and C/N ratio high.
(7) JPN PAT APPLN KOKAI PUBLICATION No. 02-281414 proposes a stacking layer medium for horizontal recording use which has an alternate structure having a cobalt-based alloy magnetic film including platinum or nickel in the alloy and a non-magnetic spacer film with a recording layer structure comprising more than one recording layer and one spacer film and, by doing so, improving an S/N.
(8) JPN PAT APPLN KOKAI PUBLICATION No. 04-60918 proposes a vertical magnetic recording medium provided by forming a CoCrNi film on a Cr film and a CoCr-based alloy film on the CoCrNi film and, by doing so, making a playback output larger and the medium lower in noise level.
(9) JPN PAT APPLN KOKAI PUBLICATION No. 04-60917 proposes a vertical magnetic recording medium provided by forming a CoCrNi film on a substrate and a CoCr-based alloy film on the CoCrNi film and, by doing so, making a playback output larger.
(10) JPN PAT APPLN KOKAI PUBLICATION No. 04-133306 proposes the method for manufacturing a vertical magnetization film by bombarding, with ions, an artificial lattice film manufactured by alternately arranging a Co layer and Pt and/or Pd layer in a stacked way and, by doing so, satisfying the coercive force and square-loop ratio.
(11) JPN PAT APPLN KOKAI PUBLICATION No. 04-189737 proposes a magnetic recording medium which, in order to make its S/N ratio larger and obtain a better overwriting a characteristic, uses two layers as a magnetic layer structure and utilizes their different magnetization inversion mechanisms.
In the above-mentioned known documents, discussions have been made on the respective magnetic layer layout of the stacked layer structure in terms of the coercive force, output and noise.
The inventors have variously studied the magnetic recording media having those magnetic recording layers of such a multilevel structure, that is, studied such magnetic receding layers on the practical disk apparatuses and found that the data is less likely to be played back after a few years. They have studied the noise in the media probably resulting from one of these reasons and found that steady-state noise is very small immediately after the data has been recorded on the media but that noise increases with the lapse of time and becomes very larger after the passage of a few years. This phenomenon is probably ascribable to the thermal fluctuation in the medium as set out above. Further it has also been found that this phenomenon markedly occurs in particular in those magnetic recording media of a multi-level (multi-layered) structure.
For the above-mentioned magnetic recording media, particularly those having a multi-layered structure for achieving the lowering of the noise, there were the problem that the degeneration of the characteristic arises with the passage of time due to an effect of noise caused by the thermal fluctuation involved.